derp/derpserver: split off derp.Server out of derp into its own package

This exports a number of things from the derp (generic + client) package
to be used by the new derpserver package, as now used by cmd/derper.

And then enough other misc changes to lock in that cmd/tailscaled can
be configured to not bring in tailscale.com/client/local. (The webclient
in particular, even when disabled, was bringing it in, so that's now fixed)

Fixes #17257

Change-Id: I88b6c7958643fb54f386dd900bddf73d2d4d96d5
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

derp/derpserver/handler.go

@@ -0,0 +1,109 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package derpserver
+
+import (
+	"fmt"
+	"log"
+	"net/http"
+	"strings"
+
+	"tailscale.com/derp"
+)
+
+// Handler returns an http.Handler to be mounted at /derp, serving s.
+func Handler(s *Server) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		ctx := r.Context()
+
+		// These are installed both here and in cmd/derper. The check here
+		// catches both cmd/derper run with DERP disabled (STUN only mode) as
+		// well as DERP being run in tests with derphttp.Handler directly,
+		// as netcheck still assumes this replies.
+		switch r.URL.Path {
+		case "/derp/probe", "/derp/latency-check":
+			ProbeHandler(w, r)
+			return
+		}
+
+		up := strings.ToLower(r.Header.Get("Upgrade"))
+		if up != "websocket" && up != "derp" {
+			if up != "" {
+				log.Printf("Weird upgrade: %q", up)
+			}
+			http.Error(w, "DERP requires connection upgrade", http.StatusUpgradeRequired)
+			return
+		}
+
+		fastStart := r.Header.Get(derp.FastStartHeader) == "1"
+
+		h, ok := w.(http.Hijacker)
+		if !ok {
+			http.Error(w, "HTTP does not support general TCP support", 500)
+			return
+		}
+
+		netConn, conn, err := h.Hijack()
+		if err != nil {
+			log.Printf("Hijack failed: %v", err)
+			http.Error(w, "HTTP does not support general TCP support", 500)
+			return
+		}
+
+		if !fastStart {
+			pubKey := s.PublicKey()
+			fmt.Fprintf(conn, "HTTP/1.1 101 Switching Protocols\r\n"+
+				"Upgrade: DERP\r\n"+
+				"Connection: Upgrade\r\n"+
+				"Derp-Version: %v\r\n"+
+				"Derp-Public-Key: %s\r\n\r\n",
+				derp.ProtocolVersion,
+				pubKey.UntypedHexString())
+		}
+
+		if v := r.Header.Get(derp.IdealNodeHeader); v != "" {
+			ctx = IdealNodeContextKey.WithValue(ctx, v)
+		}
+
+		s.Accept(ctx, netConn, conn, netConn.RemoteAddr().String())
+	})
+}
+
+// ProbeHandler is the endpoint that clients without UDP access (including js/wasm) hit to measure
+// DERP latency, as a replacement for UDP STUN queries.
+func ProbeHandler(w http.ResponseWriter, r *http.Request) {
+	switch r.Method {
+	case "HEAD", "GET":
+		w.Header().Set("Access-Control-Allow-Origin", "*")
+	default:
+		http.Error(w, "bogus probe method", http.StatusMethodNotAllowed)
+	}
+}
+
+// ServeNoContent generates the /generate_204 response used by Tailscale's
+// captive portal detection.
+func ServeNoContent(w http.ResponseWriter, r *http.Request) {
+	if challenge := r.Header.Get(NoContentChallengeHeader); challenge != "" {
+		badChar := strings.IndexFunc(challenge, func(r rune) bool {
+			return !isChallengeChar(r)
+		}) != -1
+		if len(challenge) <= 64 && !badChar {
+			w.Header().Set(NoContentResponseHeader, "response "+challenge)
+		}
+	}
+	w.Header().Set("Cache-Control", "no-cache, no-store, must-revalidate, no-transform, max-age=0")
+	w.WriteHeader(http.StatusNoContent)
+}
+
+func isChallengeChar(c rune) bool {
+	// Semi-randomly chosen as a limited set of valid characters
+	return ('a' <= c && c <= 'z') || ('A' <= c && c <= 'Z') ||
+		('0' <= c && c <= '9') ||
+		c == '.' || c == '-' || c == '_' || c == ':'
+}
+
+const (
+	NoContentChallengeHeader = "X-Tailscale-Challenge"
+	NoContentResponseHeader  = "X-Tailscale-Response"
+)